Alterations in neuronal signaling underlie a variety of forms of synaptic plasticity associated with learning and memory, and have important roles in pathological states such as epilepsy, Alzheimer's disease, Parkinson's disease and the genetically complex neuropsychiatric disorders. With the completion of the C. elegans, Drosophila and human genome sequences, neurobiologists are now able to examine the complete set of ion channels and synaptic proteins that govern neural function. Interpreting this wealth of sequence data to understand how these proteins specify the distinctive signaling properties of neurons and enable them to interconnect into computational circuits that dictate behavior will be major goals for the next decade. To this end we are interested in elucidating mechanisms that lead to abnormal neuronal hyperexcitability/seizures and analyzing alterations in gene expression that result from seizure activity to elucidate downstream pathways that might alter neuronal function or connectivity. We have conducted a large-scale behavioral screen for temperature-sensitive (TS) paralytic mutations in Drosophila that result in neuronal hyperexcitability. One complementation group identified in this screen disrupts a novel SH3-containing synaptic protein that is widely conserved throughout evolution. In addition to alterations in neuronal excitability, these mutants dramatically increase synaptic proliferation, resulting in a large increase in bouton number. The mutants are also severely learning defective in olfactory memory tests, suggesting a role for the protein in synaptic organization that is important for memory formation. We have discovered that the protein interacts with DLG, the fly PSD-95 homolog. We hypothesize that this novel protein participates in Fas II- dependent changes in synapse stabilization and Shaker-dependent changes in neuronal excitability. We will test the hypothesis that these mutants cause seizures by disrupting synapse organization and potassium channel distribution using genetic, morphological and biochemical approaches. We will also perform expression profiling using DNA microarrays to quantitate genome-wide changes in gene expression resulting from altered neuronal connectivity and activity in these lines. Experimental manipulation of the expression of these activity-regulated genes in Drosophila will allow us to examine the contribution of each gene product to potential alterations in neuronal morphology or function and may lead to new molecular tools to prevent abnormal epileptic activity.